DE69204966T2 - Thermal dye transfer printing process to make a copy of medical diagnoses. - Google Patents

Description

1. Field of the invention

The present invention relates to a dye sublimation thermal transfer process, in particular to a dye sublimation thermal transfer process for obtaining a printed copy of a medical diagnostic image and to dye image receiving elements for use in accordance with this process.

2. General state of the art

Dye sublimation thermal transfer, also referred to as dye diffusion thermal transfer, is a recording process in which a dye donor element provided with a dye layer containing sublimable dyes that can be transferred under the influence of heat is brought into contact with a receiver sheet and is selectively heated in accordance with a pattern information signal by means of a thermal print head provided with a plurality of heat-generating resistors arranged next to one another, whereby dye from the selectively heated areas of the dye donor element is transferred to the receiver sheet and forms a pattern on the latter, the shape and density of which corresponds to the pattern and the intensity of the heat acting on the dye donor element.

A dye-donor element to be used in dye sublimation thermal transfer generally comprises a very thin support, e.g. a polyester support, covered on one side with a dye layer containing the printing dyes. Generally, an adhesive layer or substrate layer is provided between the support and the dye layer. Usually, the opposite side is covered with a lubricant layer which provides a lubricious surface along which the thermal print head can be guided without suffering abrasion. An adhesive layer may be provided between the support and the lubricant layer.

A dye-image-receiving element for use in accordance with dye-sublimation thermal transfer comprises generally a carrier, eg paper or a transparent film, which is coated with a dye image receiving layer into which the dye can diffuse more easily. An adhesive layer can be provided between the carrier and the receiving layer. A separate separating layer can also be provided on the receiving layer in order to make it easier to separate the receiving element from the donor element after the transfer has taken place.

The dye layer may be a single-color dye layer or it may contain successive, repeating areas of different colored dyes, such as cyan, magenta, yellow and possibly black. If a dye donor element containing dyes corresponding to three or more primary colors is used, a multi-color image can be obtained by carrying out the dye transfer process steps sequentially for each color.

Dye sublimation thermal transfer printing can be used, among other things, to produce printed copies of images for medical diagnostics. Although it is possible to produce such a printed copy on a reflective substrate such as paper, most applications involve producing printed copies on slides. Depending on the application, these printed copies can be monochrome, in particular monochrome black, or multi-colored.

Printed copies of images produced on slides for medical diagnostics are read in front of a light box using a light source.

A disadvantage of the known dye sublimation thermal transfer recording materials for producing printed copies of images for medical diagnostics is that when the image produced on the dye sublimation thermal slide is viewed in front of a light box, reflections from the nearby environment at small angles (eg the observer sees himself as if in a mirror, Reflections from a light source approximately behind the observer) make it difficult to interpret the actual image, especially in the high-density areas, which interferes with the diagnosis.

3. Brief description of the invention.

The object of the present invention is therefore to provide a dye thermal transfer printing process for obtaining a printed copy of a medical diagnostic image on a slide and a transparent receiver element to be used in accordance with this process, which do not have the disadvantages mentioned above.

According to the invention there is now provided a transparent dye image receiving element for use in accordance with dye thermal transfer printing, the dye receiving element comprising a support and having thereon a dye image receiving layer and optionally with at least one backing layer on the opposite side of the support, characterized in that the dye image receiving element is such that after transfer of a dye image to the receiving element the specular gloss of the transferred dye image, measured from the readable side of the image, in areas with an optical density of at least 2.00 and at most 90.

Also according to the invention there is provided a dye thermal transfer printing process in which a dye donor element comprising a support and a dye layer thereon is imagewise heated, whereby a dye image is transferred to a transparent dye image receiving element comprising a support and thereon a dye image receiving layer and optionally with at least one backing layer on the opposite side of the support, characterized in that the dye image receiving element is such that after transfer of a dye image to the receiving element the specular gloss of the dye image obtained, measured from the readable side of the image, in areas with an optical density of at least 2.00 and at most 90.

Printed copies for medical diagnostics obtained by means of the method according to the invention and/or the receiver element according to the invention have lower reflection than those obtained using the known dye thermal transfer printing methods and/or receiver elements and are therefore easier to interpret.

4. Detailed description of the invention.

Specular gloss is defined as the ratio between the reflection of the sample and the reflection of a reference material (x 100).

The specular gloss is measured according to the applicable standards ASTM D523, DIN 67530 and ISO 2813, with the angle of incidence and angle of reflection both being 20º. Highly polished, flat, black glass with a refractive index of 1.567 is used as the reference material; this reference material is assigned a gloss value of 100 for each measurement geometry (i.e. for each angle of incidence and reflection). The measurements are carried out on a glossmeter supplied by DR LANGE GmbH, Wiesenstrasse 21, 4000 Düsseldorf 11, Germany, which is called a laboratory reflectometer. The specular gloss is measured from the readable side of the image. This means that when a mirror image of the original is transferred to the receiver layer, the gloss is measured from the back of the receiver element, i.e. the side opposite the receiver layer, since the image on the printed copy is then also viewed on the light box from the back. If a readable image of the original is transferred to the receiver layer, the gloss is measured from the receiver layer side, since in this case the image on the light box is viewed from the receiver layer side.

The smaller the numerical value of the gloss measurement, the more matte the image.

According to the invention, the specular gloss of the transferred dye image is preferably at most 80, more preferably at most 70, even more preferably at most 60, even more preferably at most 50, even more preferably at most 40 and even more preferably at most 30, measured from the readable side of the image.

The measurement of the surface gloss is preferably carried out on image areas with an optical density of at least 2.10, particularly preferably on image areas with an optical density of at least 2.20.

These densities can be achieved by single printing, i.e. by printing the receiver element once together with the donor element, or by double (or multiple) printing, i.e. by printing the same receiver element a second time in register with the same area or a different area of the donor element (such printing processes have been described, for example, in EP 318946, EP 452566 and EP-A-0522207; EP-A-0522207 is part of the state of the art according to Article 54(3)(4) EPO in the contracting states BE, DE, FR, GB and NL).

The density of the image area is measured in transmitted light (specular diffuse according to ISO 5/2) using a Macbeth TR 924 densitometer operated in status A for monochrome coloured image areas (e.g. magenta, yellow or cyan images behind a green, blue or red filter) or fitted with a visual filter for black images. For black image areas obtained using monochrome black or coloured dye-donor elements, the optical density is measured in the area with a visually neutral grey colour (ie with CIE a* and b* values each in the range -8.0 to +8.0, measured according to ASTM E308 (method defined by the Commission International pour l'Eclairage)), illuminated by a standard light source such as is commonly used in negatoscopes for medical diagnostic purposes. A non-limiting example is The list below is for information purposes only: the TL lamps of the following types HLX 182, WWX 183, WWX, WW, W, CWX 184, CWX, UW, GW, N, DX 186, D, WWX 193, WX 194 (e.g. type F58W/CW-ST 133), all available from Sylvania GTE, and the following types 29, 33, 54, 82, 83, 84, 86, 92, 93, 94, 95 (e.g. type TL'D 58W 33), all available from Philips. The CIE values mentioned above are net differences related to the CIE values of the carrier of the receiver element.

According to a preferred embodiment of the present invention, one of the layers of the dye-image-receiving element contains an effective amount of a matting agent such that the specular gloss of the transferred image is within the ranges defined above.

If a legible image of the original is transferred to the receiver layer, the matting agent is mixed into the dye-image-receiving layer or into a layer provided on this dye-image-receiving layer.

If a mirror image of the original is transferred to the receiver layer, the matting agent is mixed into at least one of the backing layers provided on the side of the carrier opposite the receiver layer.

Preferably, the matting agent is mixed into at least one of the backing layers and therefore a mirror image is transferred onto the receiving layer. Mixing the matting agent into the receiving layer can lead to less close contact between donor element and receiving element during transfer. Other matting agents are often not soluble in the organic solvents in which the binder resins for the receiving layer are soluble and often do not accept the dye either. An additional advantage of mixing the matting agent into at least one of the backing layers is the better resistance of the receiving element to the drum on which the receiving element is clamped during transfer. Further incorporation of matting agent into one of the backing layers can improve the conductivity, anti-blocking and general handling properties of the receiver element so that individual receiver sheets can be easily removed from the stack of receiver sheets and fed smoothly and sequentially to the print head. Further incorporation of matting agent into one of the backing layers can also result in a more economical manufacture of the receiver sheet in that the transport of the receiver sheet over the various rollers during coating is improved and there is less sticking of the back side of one layer to the receiver side of the next layer during storage of the coated receiver in a rolled state.

According to the invention, matting agents used include homopolymers of methyl methacrylate, copolymers of methyl methacrylate and methacrylic acid, copolymers of methyl methacrylate, methacrylic acid and styrene, copolymers of methyl methacrylate, styrene and maleic acid, starch or other organic compounds and fine-grained inorganic compounds such as silica, titanium dioxide, strontium compounds, barium compounds, etc. The volume-average grain size of these matting agents is preferably 0.3 to 10 μm, particularly 0.75 to 4.5 μm, very particularly 0.75 to 2.5 μm.

If necessary, a mixture of two or more matting agents can be used.

Examples of preferred matting agents are:

- Methyl methacrylate-styrene-maleic acid copolymer (95/2.5/2.5) with a volume average diameter of 1 µm (matting agent No. 2; see the examples below)

- Methyl methacrylate-styrene-maleic acid copolymer (95/2.5/2.5) with a volume average diameter of 2 µm (matting agent No. 1; see the examples below)

- Methyl methacrylate-styrene-maleic acid copolymer (95/2.5/2.5) with a volume average diameter of 3 um

- Methyl methacrylate-stearyl methacrylic acid copolymer (98/2)

- Methyl methacrylate-styrene-maleic acid-stearyl methacrylic acid copolymer (96.55/0.74/0.74/1.97) with a volume average diameter of 3.5 µm

- Methyl methacrylate-stearyl methacrylic acid-styrene- maleic acid copolymer (96.55/1.97/0.74/0.74) with a volume-average diameter in the range of 5 to 7 um

- Methyl methacrylate-vinylbenzyl chloride-acrylic acid copolymer (90/5/5) with a volume average diameter of about 1.5 µm

- Methyl methacrylate-stearyl methacrylic acid-maleic acid copolymer (42/56/2) with a volume-average diameter in the range of 1 to 3.5 µm

- Hydroxypropylmethylcellulose hexahydrophthalate (5.5- 7%/16-18.5%/35-40.5%) (HPMC-EHP available from Shin-Etsu Chemical Company)

- Styrene-alkyl methacrylic acid copolymer with a volume-average diameter of about 0.5 µm (Ropaque OP62, available from Rohm & Haas)

- Silica with a volume average diameter of 4.1 um (Syloid 378, available from Grace Davison)

- Silica with a volume-average diameter in the range of 0.2 to 6 um (Millisil C800, available from Sibelco)

- Silica with a volume average diameter in the range of 3.8 to 4.4 µm (Syloid 72, available from Grace Davison).

The amount of matting agent is advantageously in the range of 2% to 30%, preferably 5% to 15%, based on the weight of binder resin of the layer into which the matting agent is mixed.

The polymeric binder resin of the backing layer can be any polymer resin known to the art to form a continuous, preferably uniform layer which transparent and capable of adhering to the substrate.

Possible polymeric binders are: gelatin (possibly modified), starch, dextrans (possibly modified), polycarbonates, cellulose esters, cellulose ethers, homo- and copolyesters, vinyl polymers (e.g. polyvinyl alcohol, polyvinyl acetate, polyvinyl chloride), vinyl copolymer (e.g. vinyl chloride- vinyl acetate-vinyl alcohol copolymer), (meth)acrylic polymers such as copolymers of acrylic acid and/or methacrylic acid and/or their lower (C₁-C₆)alkyl esters (e.g. copolymers of ethyl acrylate and methyl methacrylate, copolymers of methyl methacrylate/butyl acrylate/acrylic acid, typically in the molar ratios 55/27/18% and 36/24/40%, and in particular hydrophilic functional groups containing copolymers such as copolymers of methyl methacrylate and methacrylic acid, as well as crosslinkable copolymers which contain, for example, molar ratios of around 46/46/8% of ethyl acrylate/methyl methacrylate/acrylamide or methacrylamide), styrene copolymers (e.g. styrene-acrylonitrile copolymer), polyurethanes, polyamides and polyimides.

The backing layer may also contain a cross-linked binder cured by the action of heat or UV or electron beams (e.g. the heat-cured product of vinyl chloride-vinyl acetate-vinyl alcohol copolymer and polyisocyanate).

Electron-conductive polymers can also be used as binders for the backing layer. Examples of such polymers are described in Chapter 3 of "Introduction to synthetic electrical conductors", Academic Press, 1987. From among these conductive polymers, polyanilines, polythiophenes (as described in EP 440957 and DE 4003720) and polypyrrole and their derivatives are particularly recommended.

Mixtures with one or more of the polymers listed above can also be used as binders for the backing layer.

The thickness of the backing layer(s) can vary over a wide range, but is generally in the range of 1 to 10 µm. Some of the matting agent particles may protrude from the free surface of the backing layer(s). Therefore, a thickness of the backing layer(s) of approximately 1 to 4 µm is recommended.

The backing layer(s) may optionally contain an additive, for example an antistatic agent (such as a copolymer of potassium acrylate, acrylic acid, methacrylate and tetraallyloxyethane), antioxidant, stabilizer, plasticizer, dispersant, wetting agent (such as sodium 1-isobutyl-6-methyloctyl sulfate and ammonium perfluorodecyl carboxylate).

If more than one backing layer is provided on the opposite side of the carrier, the matting agent is preferably contained in the outermost backing layer. Antistatic agents can then, for example, be mixed into the innermost backing layer.

The formation of the backing layer(s) can be carried out by methods known to those in the art, whereby the application of the layer to the substrate supporting it is advantageously carried out from a coating composition containing a solution or dispersion of the binder resin and other components (such as the matting agent) in a volatile medium.

Since most of the above-mentioned matting agents are insoluble in water, aqueous coating media can be used provided that the polymeric binder of the film formation is capable of providing a continuous, uniform coating, generally when applied from an aqueous solution, dispersion or aqueous latex. Alternatively, the volatile liquid medium can be a commonly used organic solvent or solvent mixture in which the polymeric binder is soluble and is also such that the matting agent particles do not dissolve in the coating composition. Suitable organic solvents include methanol, acetone, ethanol and methyl ethyl ketone. Such Solvents may be mixed with small amounts of other solvents such as methylene chloride, diacetone alcohol and methoxypropan-2-ol.

The applied coating medium is then dried to remove the volatile medium and, if necessary, to effect cross-linking of the binder components. Drying can be carried out by conventional methods, for example by passing the coated film support through a hot air oven. Of course, drying can be carried out during conventional post-film formation treatments such as heat setting.

The formation of the backing layers by application of a liquid coating composition can be carried out at any suitable stage in the manufacture of the receiver sheet.

The dye-receiving layer of the receiver element according to the invention can contain, as a binder, which must be transparent, for example a polycarbonate, a polyurethane, a polyester, a polyamide, polyvinyl chloride, styrene-acrylonitrile copolymer, polycaprolactone or mixtures thereof. Suitable dye-receiving layers have been described, for example, in EP 133011, EP 133012, EP 144247, EP 227094 and EP 228066. The dye-receiving layer can also contain a hardened binder such as the heat-hardened product of vinyl chloride-vinyl acetate-vinyl alcohol copolymer and polyisocyanate.

The total amount of binder used in the dye-receiving layer according to the invention is between 25 and 95 percent by weight, preferably between 50 and 80 percent by weight.

The dye-receiving element according to the invention can contain a release agent to improve the release properties with respect to the donor element. Suitable release agents are solid waxes such as polyethylene wax, amide wax and Teflon powder; fluorine-based and phosphate ester-based surfactants; and paraffin-based, silicone-based and fluorine-based oils. Silicone oils are preferred. in particular reactive silicone oils (such as hydroxyl-modified polydimethylsiloxane, e.g. TEGOMER HSI 2111, available from Goldschmidt), as well as silicone-containing copolymers such as polysiloxane-polyether copolymers and block copolymers (e.g. TEGOGLIDE, available from Goldschmidt, and SILWET, available from Union Carbide).

High-boiling organic solvents or thermal solvents or plasticizers can also be mixed into the image receiving layer, namely as substances that can absorb or dissolve the dyes or as diffusion accelerators for the dyes. Suitable high-boiling organic solvents and thermal solvents of this type include the compounds known from JP 62/174754, JP 62/245253, JP 62/30247 and JP 62/136646.

Also, to further improve the lightfastness of the transferred image, one or two or more types of additives such as UV absorbers, light stabilizers and antioxidants may optionally be added. The amounts of these UV absorbers and light stabilizers are preferably 0.05 to 10 parts by weight and 0.5 to 15 parts by weight, respectively, per 100 parts of the resin constituting the receiving layer.

The dye-receiving layer according to the invention preferably has a total thickness of 0.5 to 50 µm, particularly preferably of 2.5 to 10 µm.

If a cover layer containing a release agent of the type described above is provided on the receiving layer, the thickness of such a cover layer is preferably 0.01 to 5 µm, in particular 0.05 to 2 µm.

A transparent film or sheet made of various plastics such as polyethylene terephthalate, polyolefin, polyvinyl chloride, polystyrene, polycarbonate, polyethersulfone, polyimide, cellulose ester or vinyl alcohol acetal copolymer is used as a carrier for the receiver sheet. Blue-colored Polyethylene terephthalate film may be used as long as it remains transparent, transparent being defined as the property of allowing light to pass through without significant scattering. In general, the support has a thickness of at least 100 µm so that the printed copy can be easily mounted on a light box. The thickness of the support is preferably in the range of 120 to 200 µm, more preferably in the range of 160 to 190 µm, most preferably between 170 and 180 µm.

The adhesion of a coating composition to the carrier material can be improved by providing a substrate layer between the carrier material and the coating layer (e.g. the receiving layer and/or the backing layer). Particularly preferred as substrate layers for polyethylene terephthalate carriers are substrate layers based on vinylidene chloride copolymers, as described in GB 1234755.

The image-receiving element according to the invention can also comprise an intermediate layer between the support and the image-receiving layer. Depending on the material from which they are formed, the intermediate layers can function as support layers, porous layers (as long as they remain transparent) or dye diffusion-preventing layers, or two or more of these functions, but can also serve as an adhesive, depending on the particular use.

The material constituting the intermediate layer can contain, for example, a urethane resin, an acrylic resin, an ethylenic resin, a butadiene rubber or an epoxy resin. The thickness of the intermediate layer is preferably 1 to 20 µm.

Dye diffusion-preventing layers are layers which prevent the dye from diffusing into the support. The binders used to form these layers can be water-soluble or soluble in organic solvents, but the use of water-soluble binders is preferred, with gelatin being particularly is attractive.

Porous layers are layers that prevent the diffusion of heat supplied at the time of thermal transfer from the image receiving layer into the carrier in order to ensure that the heat supplied is used efficiently and to prevent any deformation of the carrier.

The front or rear surface of the image receiving element according to the invention can also be subjected to an antistatic treatment. Such an antistatic treatment can be carried out by incorporating an antistatic agent, for example, into the image receiving layer, or can consist of an antistatic layer applied to the image receiving surface or inserted under it. The rear surface can also be subjected to a similar treatment. Such a treatment allows the image receiving sheets to slide easily along one another and also results in no dust being able to adhere to the image receiving sheet.

Furthermore, the image receiving sheet can have a transparent sliding layer provided on the back surface of the film carrier. The material for the sliding layer can comprise methacrylate resins such as methyl methacrylate etc. or corresponding acrylate resins, vinyl resins such as vinyl chloride-vinyl acetate copolymers.

Furthermore, the receiver element can have a notch to distinguish the receiver layer side from the back layer side.

A dye-donor element to be used in accordance with dye-sublimation thermal transfer in conjunction with the present receiver element usually comprises a very thin carrier, e.g. a polyester carrier, one side of which is covered with a dye layer containing the printing dyes. Usually an adhesive layer or substrate layer is provided between the carrier and the dye layer. Normally the opposite side is covered with a lubricant layer which provides a slippery surface to which along which the thermal print head can be guided without suffering abrasion. An adhesive layer can be provided between the carrier and the lubricant layer.

The dye layer may be a single-color dye layer or it may contain successive, repeating areas of different colored dyes, such as cyan, magenta, yellow and possibly black. If a dye donor element containing dyes corresponding to three or more primary colors is used, a multi-color image can be obtained by carrying out the dye transfer process steps sequentially for each color.

The dye layer of such a dye-sublimation thermal transfer donor element is preferably formed by adding the dyes, the polymeric binder and other optional components to a suitable solvent or solvent mixture, dissolving or dispersing the components to form a coating composition which is then applied to a support which may first have been provided with an adhesive layer or substrate layer and then dried.

The dye layer thus formed has a thickness of about 0.2 to 5.0 µm, preferably 0.4 to 2.0 µm, and the ratio of dye to binder is between 9:1 and 1:3 by weight, preferably between 2:1 and 1:2 by weight.

Suitable polymeric binders are: cellulose derivatives such as ethyl cellulose, hydroxyethyl cellulose, ethyl hydroxycellulose, ethyl hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, nitrocellulose, cellulose acetate formate, cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate pentanoate, cellulose acetate benzoate, cellulose triacetate; resins and derivatives of the vinyl type such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, Polyvinyl butyral-vinyl acetal-vinyl alcohol copolymer, polyvinylpyrrolidone, polyvinyl acetoacetal, polyacrylamide; polymers and copolymers derived from acrylates and acrylate derivatives such as polyacrylic acid, polymethyl methacrylate and styrene-acrylate copolymers; polyester resins; polycarbonates such as a polycarbonate derived from 2,2-bis-(4-hydroxyphenyl)propane; styrene-acrylonitrile copolymer; polysulfones, polyphenylene oxide; organosilicone such as polysiloxanes; epoxy resins and natural resins such as gum arabic. Cellulose acetate butyrate or styrene-acrylonitrile(butadiene) copolymer is preferably used as a binder for the dye layer.

Any dye can be used in such a dye layer, provided that it can be readily transferred to the dye image receiving layer of the receiver sheet by the action of heat.

Typical and specific examples of dyes to be used in dye sublimation thermal transfer are described, for example, in EP 485665, EP 209990, EP 209991, EP 216483, EP 218397, EP 227095, EP 227096, EP 229374, EP 235939, EP 247737, EP 257577, EP 257580, EP 258856, EP 279330, EP 279467, EP 285665, EP 400706, EP 432313, EP 432314, EP 432829, EP 453020, US 4743582, US 4753922, US 4753923, US 4757046, US 4769360, US 4771035, JP 84/78894, JP 84/78895, JP 84/78896, JP 84/227490, JP 84/227948, JP 85/27594, JP 85/30391, JP 85/229787, JP 85/229789, JP 85/229790, JP 85/229791, JP 85/229792, JP 85/229793, JP 85/229795, JP 86/41596, JP 86/268493, JP 86/268494, JP 86/268495 and JP 86/284489.

The coating layer may also contain other additives such as hardeners, preservatives, organic or inorganic fine grains, dispersants, antistatic agents, antifoam agents, viscosity regulators, etc., which, along with other ingredients, are described in more detail in EP 133011, EP 133012, EP 111004 and EP 279467.

As a carrier for the dye donor element Any material may be used provided that it is dimensionally stable and can withstand the temperatures involved, up to 400ºC for a period of up to 20 msec, and yet is thin enough that heat applied to one side is transmitted to the dye on the other side to effect transfer to the receiver sheet within such short periods of time, typically 1 to 10 msec. Such materials include polyesters such as polyethylene terephthalate, polyamides, polyacrylates, polycarbonates, cellulose esters, fluorinated polymers, polyethers, polyacetals, polyolefins, polyimides, glassine paper and capacitor paper. A polyethylene terephthalate support is preferred. Generally the support has a thickness of 2 to 30 µm. Optionally the support may also be coated with an adhesive layer or substrate layer.

The dye layer of the dye donor element can be applied to the support or printed using a printing technique, such as a gravure printing process.

The dye-donor element may also have a dye barrier layer comprising a hydrophilic polymer inserted between the support and the dye layer to improve dye transfer densities by preventing transfer of dye in the wrong direction, towards the support. The dye barrier layer may comprise any hydrophilic material suitable for the intended purpose. In general, good results have been obtained with gelatin, polyacrylamide, polyisopropylacrylamide, butyl methacrylate grafted gelatin, ethyl methacrylate grafted gelatin, ethyl acrylate grafted gelatin, cellulose monoacetate, methyl cellulose, polyvinyl alcohol, polyethyleneimine, polyacrylic acid, a mixture of polyvinyl alcohol and polyvinyl acetate, a mixture of polyvinyl alcohol and polyacrylic acid, or a mixture of cellulose monoacetate and polyacrylic acid. Suitable dye barrier layers have been described in, for example, EP 227091 and EP 228065. Certain hydrophilic Polymers, eg those described in EP 227091, also exhibit sufficient adhesion to the support and dye layer, thereby eliminating the need for a separate adhesive or substrate layer. These particular hydrophilic polymers used in a single layer in the donor element thus perform a dual function and are therefore referred to as dye barrier/substrate layers.

Preferably, the back of the dye donor element can be coated with a lubricant layer to prevent the print head from sticking to the dye donor element. Such a lubricant layer would contain a lubricant medium such as a surfactant, a liquid lubricant, a solid lubricant or mixtures thereof, optionally with a polymeric binder. The surfactants can be any of those known in the art, such as carboxylates, sulfonates, phosphates, aliphatic amine salts, aliphatic quaternary ammonium salts, polyoxyethylene alkyl ethers, polyethylene glycol fatty acid esters, aliphatic fluoroalkyl C2-C20 acids. Examples of liquid lubricants include silicone oils, synthetic oils, saturated hydrocarbons and glycols. Examples of solid lubricants include various higher alcohols such as stearyl alcohol, fatty acids and fatty acid esters. Suitable lubricant layers are described, for example, in EP 138483, EP 227090, US 4567113, US 4572860 and US 4717711. Preferably, the lubricant layer contains a styrene-acrylonitrile copolymer or a styrene-acrylonitrile-butadiene copolymer or a cellulose ester or a polycarbonate derived from 2,2-bis-(4-hydroxyphenyl)propane as a binder and a polysiloxane-polyether copolymer or polytetrafluoroethylene as a lubricant in an amount of 0.1 to 10 percent by weight of the binder (mixture).

The dye layer of the dye donor element may also contain a release agent which facilitates the separation of the dye donor element from the dye acceptor element after transfer. The release agents can be applied in a separate layer to at least part of the dye layer. Solid waxes, fluorine- or phosphate-containing surfactants and silicone oils are used as release agents. Suitable release agents are described in EP 133012, JP 85/19138 and EP 227092, for example.

The ink receiving elements according to the invention are used to form a dye transfer image. In such a process, the dye layer of the donor element is placed against the dye receiving layer of the receiver sheet so that their surfaces touch, whereupon imagewise heating takes place from the back of the donor element. The transfer of the dye takes place by heating for several milliseconds to a temperature of 400°C.

If the process is carried out for only a single color, a single color dye transfer image is obtained. A multi-color image can be obtained by using a donor element containing three or more primary color dyes and sequentially performing the above-described process steps for each color. This layered arrangement of donor element and receiver sheet is formed three times within the period during which heat is applied via the thermal print head. After the first dye is transferred, the elements are released from each other. A second dye donor element (or another area of the donor element with a different dye area) is then brought into register with the dye receiver element and the process is repeated. The third color and optionally additional colors are obtained in the same way.

In addition to thermal heads, laser light, infrared flashes or heated pens can also be used as a heat source for supplying heat energy. Thermal print heads that can be used to transfer dye from the dye donor element to the receiver sheet are commercially available. When using laser light, the dye layer or another layer of the Dye element may contain a compound that absorbs the light emitted by the laser and converts it into heat, e.g. soot.

The carrier of the dye donor element can also be an electrically poorly conductive ribbon, which consists, for example, of a multi-layer system of carbon-loaded polycarbonate coated with a thin aluminum film. Current is introduced into the poorly conductive ribbon by electrically charging a print head electrode, which leads to highly localized heating of the ribbon under the relevant electrode. The fact that the heat is generated directly in the poorly conductive ribbon and that the ribbon is thus heated provides an inherent advantage of the technology using a combination of poorly conductive ribbon and electrode head over thermal head technology, where the various elements of the thermal head must heat up and cool down before the head can move on to the next printing position.

The method according to the invention is used for producing printed copies, in particular black and white printed copies of images for medical diagnostics, in particular in ultrasound, C-arm surgery and nuclear medicine applications. The following examples may explain the invention in even more detail, but without restricting its scope.

EXAMPLES

A dye donor element was prepared as follows:

A solution was prepared from 8 wt.% dye A, 4.8 wt.% dye B, 4 wt.% dye C, 8 wt.% styrene-acrylonitrile copolymer as binder and 2.5 wt.% octanediol as thermal solvent in methyl ethyl ketone as solvent. This solution was used to apply a layer with a wet thickness of 10 µm to a 5 µm thick polyethylene terephthalate film. The layer thus formed was removed by evaporation of the solvent dried. Dye A Dye B Dye C

The back of the polyethylene terephthalate film was provided with a sliding layer consisting of a 13 wt. % styrene-acrylonitrile copolymer as a binder and 1 wt. % polysiloxane-polyether copolymer as a lubricant in methyl ethyl ketone as a solvent.

Dye-imager elements were prepared as follows:

A blue-colored polyethylene terephthalate film of 175 µm, provided on both sides with a substrate layer with vinylidene chloride copolymer, was coated with a composition consisting of 94 g of vinyl chloride-vinyl acetate-vinyl alcohol copolymer (91/3/6 wt. %) (available under the trade name VINYLITE VAGD from Union Carbide, Old Ridgeburry Road, Danbury, USA) and 13 g of diphenylmethane-4,4'-diisocyanate (available under the trade name DESMODUR VL from Bayer, Leverkusen, Germany) dissolved in 893 g of methyl ethyl ketone to form the receiving layer (wet layer thickness 40 µm). The dry weight of the dye image receiving layer thus obtained was 4.3 g/m². A release layer was produced on this layer. This release layer was coated with a wet layer thickness of 31 µm using a mixture of 13.7 g A composition consisting of vinyl chloride-vinyl acetate-vinyl alcohol copolymer (91/3/6 wt. %) (available under the brand name VINYLITE VAGD from Union Carbide) and 7.6 g of a hydroxyl-modified polydimethylsiloxane (available under the brand name TEGOMER HSI 2111 from Th. Goldschmidt AG, Goldschmidtstrasse 100, Essen, Germany) and 0.11 g of dibutyltin laurate (available under the brand name STAVINOR 1250 SN from Rousselot Sa, Rue Christophe Colomb, Paris, France) as a catalyst, dissolved in 978 g of methyl ethyl ketone, was applied. The dry weight of the release coating thus obtained was 0.69 g/m². After application, the layers were dried and heat-set.

On the opposite side of the support, a backing layer was prepared by bead coating using an aqueous solution (wet film thickness 30 µm). The coating composition contained: gelatin (37 g/l), formaldehyde as a hardener (4% by weight based on the gelatin), wetting agent and a matting agent, the type and amount of which are given in Table 1 below. Table 1 Example Matting Agent Quantity (g/m²)

(a) In Examples 7 and 8, the backing layer of the receiving element contains 0.33 g/m² of an antistatic agent (a copolymer of potassium acrylate, acrylic acid, methacrylic acid and tetraallyloxyethane). Furthermore, in Examples 7 and 8, another donor element with different dye compositions, namely 8 wt.% of dye A, 2.4 wt.% of dye B and 6.4 wt.% of dye D). Dye D

The dye-receiving element thus obtained was printed together with the dye-donor element in a Mitsubishi video printer of the type CP 100E.

The receiver sheet was separated from the dye-donor element and the specular gloss of the resulting black-and-white image was measured from the backing side in an area of visually neutral grey colour and an optical density as shown in Table 2 below. The specular gloss was measured at a value of 20º for both the angle of incidence and the angle of reflection using a laboratory reflectometer supplied by DR LANGE. The optical density was measured using a Macbeth TR 924 densitometer equipped with a visual filter.

The results are presented in Table 2 below. Table 2 Example Density Gloss

In all of the images obtained above, unwanted reflections occurred in areas of high Density when viewed on a light box is significantly lower than that of images obtained using commercially available dye transfer printing materials.

Claims (19)

1. A transparent dye-image-receiving element for use in accordance with dye thermal transfer printing, the dye-receiving element comprising a support and thereon a dye-image-receiving layer and optionally with at least one backing layer on the opposite side of the support, characterized in that the dye-image-receiving element is such that after transfer of a dye image to the receiving element, the specular gloss of the transferred dye image, measured from the readable side of the image, in areas with an optical density of at least 2.00 is at most 90. 2. A transparent dye-image-receiving element according to claim 1, wherein the dye-image-receiving element is such that after transfer of a dye image to the receiver element, the specular gloss of the transferred dye image, measured from the readable side of the image, is at most 50 in areas having an optical density of at least 2.00. 3. A transparent dye-image-receiving element according to claim 2, wherein the dye-image-receiving element is such that after transfer of a dye image to the receiver element, the specular gloss of the transferred dye image, measured from the readable side of the image, is at most 30 in areas having an optical density of at least 2.00. 4. A transparent dye-image-receiving element according to any preceding claim, wherein the specular gloss is measured in areas having an optical density of at least 2.20. 5. A transparent dye-image-receiving element according to any of the preceding claims, wherein the optical density is achieved by single printing. 6. A transparent dye-image-receiving element according to any of the preceding claims, wherein a layer of the dye-image-receiving element contains a matting agent. 7. Transparent dye-image-receiving element according to Claim 6, wherein the volume average grain size of the matting agent is 0.75 to 4.5 µm. 8. A transparent dye-image-receiving element according to claim 6 or 7, wherein the matting agent is silica or a copolymer of methyl methacrylate, styrene and maleic acid. 9. A transparent dye-image-receiving element according to any of claims 6, 7 and 8, wherein the amount of matting agent is between 5 and 15% by weight of the binder resin. 10. A transparent dye-image-receiving element according to any of claims 6 to 9, wherein the matting agent is contained in at least one of the backing layers. 11. A transparent dye-image-receiving element according to claim 10, wherein the binder resin of the backing layer is gelatin. 12. A transparent dye-image-receiving element according to claim 10 or 11, wherein at least one of the backing layers contains an antistatic agent. 13. A transparent dye-image-receiving element according to any of the preceding claims, wherein the support is transparent uncolored polyethylene terephthalate or transparent blue colored polyethylene terephthalate. 14. A transparent dye-image-receiving element according to any of the preceding claims, wherein the support has a thickness of at least 120 µm. 15. A transparent dye-image-receiving element according to claims 13 or 14, wherein the support is provided on one side or on both sides with a substrate layer containing a copolymer of vinylidene chloride. 16. A thermal dye transfer printing process in which a dye donor element comprising a support and a dye layer thereon is imagewise heated, whereby a dye image is transferred to a transparent dye image receiving element, characterized in that the dye image receiving element is one as defined in any of the preceding claims. 17. A dye thermal transfer printing method according to claim 16, wherein the method is used to obtain a printed copy of a medical diagnostic image. 18. The dye thermal transfer printing method according to claim 17, wherein the printed copies are black and white printed copies. 19. Dye thermal transfer printing process according to one of claims 16 to 18, wherein the heating is carried out by laser light.